NOAA Technical Memorandum NMFS 

AUGUST 2005 

CONTRACTION OF THE SOUTHERN RANGE LIMIT 

FOR ANADROMOUS ONCORHYNCHUS MYKISS

David A. Boughton 

Heidi Fish 

Kerrie Pipal 

Jon Goin 

Fred Watson 

Julie Casagrande 

Joel Casagrande 

Matt Stoecker 

NOAA-TM-NMFS-SWFSC-380 

U.S. DEPARTMENT OF COMMERCE 

National Oceanic and Atmospheric Administration 

National Marine Fisheries Service 

Southwest Fisheries Science Center 
NOAA Technical Memorandum NMFS 

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Documents within this series, however, reflect sound professional work and may 

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NOAA Technical Memorandum NMFS 

This TM series is used for documentation and timely communication of preliminary results, interim reports, or special 

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AUGUST 2005 

CONTRACTION OF THE SOUTHERN RANGE LIMIT 

FOR ANADROMOUS ONCORHYNCHUS MYKISS

David A. Boughton1, Heidi Fish1, Kerrie Pipal1, Jon Goin1, Fred Watson2, 

Julie Casagrande2

, Joel Casagrande2, and Matt Stoecker3 

1

 Fisheries Ecology Division 

Southwest Fisheries Science Center 

NOAA National Marine Fisheries Service 

110 Shaffer Road 

Santa Cruz, CA 95060 

2

 The Watershed Institute 

California State Universtiy Monterey Bay 

100 Campus Center 

Seaside, CA 93955 

3

 Stoecker Ecological 

P.O. Box 2062 

Santa Barbara, CA 93120 

NOAA-TM-NMFS-SWFSC-380 

U.S. DEPARTMENT OF COMMERCE 

Carlos M. Gutierrez, Secretary 

National Oceanic and Atmospheric Administration 

Vice Admiral Conrad C. Lautenbacher, Jr., Under Secretary for Oceans and Atmosphere 

National Marine Fisheries Service 

William T. Hogarth, Assistant Administrator for Fisheries 

 

 

Abstract 

For many species, contraction of the geographic range limit nearest the equator is predicted by current global climate trends. Anecdotal data suggest that the southern range 

limit for anadromous populations of the fish Oncorhynchus mykiss has recently contracted 

northward; the contraction is consistent with the climate-change hypothesis, but is also 

consistent with the coincidental urbanization of the species’ southern range limit during 

the twentieth century. Distinguishing between the two mechanisms of range contraction 

is important in part to plan conservation measures for the species; and in part to accurately account for the impacts of climate change vs other impacts on species’ geographic 

distributions. Here we assess the current occurrence of anadromous O. mykiss in each 

coastal basin of southern California in which it occurred historically. The extent of the 

study is the region from Monterey Bay (the Pajaro River basin) southward to the US Border with Mexico; the grain of the study (the smallest unit for which occurrence is estimated) is the coastal basin. Two especially large basins—those of the Pajaro and Salinas 

rivers—were divided into sub-units. Ninety-two historic steelhead basins were identified, and occurrence was estimated in 86 of them through a combination of field reconnaissance and spot checks (snorkel surveys). The spot checks were conducted in the bestoccurring habitat of stream reaches with migration-connectivity to the ocean. The results 

indicated that between 58% and 65% of historic steelhead basins currently harbor O. 

mykiss populations. The pattern of basin-level extirpation showed a latitudinal gradient, 

with the highest extirpation rates in the south, confirming that a range contraction has 

taken place. Most of the extirpations (68%) were associated with anthropogenic barriers 

to fish migration (dams, flood-control structures, culverts, etc.). Historically, the probability of occurrence in a basin was correlated with basin area, but this correlation no longer 

holds. Barrier-associated extirpations were positively correlated with urban/agricultural 

development, whereas the “other” extirpations (i.e,. not associated with barriers) were 

positively correlated with mean annual air temperature in the basin. The results suggest 

that the range contraction is primarily an effect of lost migration opportunities arising 

from the urban/rural infrastructure; however climate and other stressors may have 

played a subsidiary role. 

 Contraction of the southern range limit 1 

 

Introduction 

Large-scale range contractions of species are 

notable events. One reason for this is that range 

contraction on the equatorial side of a species 

range is considered to be evidence for biologicallysignificant climate warming (Kareiva et al. 1993, 

Easterling et al. 2000). This sort of global warming 

in the twentieth century has been welldocumented (Houghton 1997), and range shifts 

consistent with global warming have been observed in climatically sensitive species such as butterflies (Parmesan et al. 1999), even as the uniqueness of the recent warming trend itself is vigorously debated (Mann et al. 1998, McIntyre and 

McKitrick 2003). 

In fact range contractions may occur for any 

number of non-climatic reasons as well—broadscale habitat alteration may cause population extinctions throughout a species’ range, with the 

effect of shrinking the spatial envelope encircling 

the extant populations; or local-scale impacts of 

human activity may happen to coincide geographically with the species’ range limit, driving a 

range contraction. Whatever the primary cause, 

range contractions on the order of thousands of 

square kilometers are widely considered to foreshadow species extinction. The redlist of the 

World Conservation Union (IUCN) describes significant declines in area of occurrence to be risk 

criteria (Musick 1999), and the US Endangered 

Species Act includes extinction in a significant 

portion of the range as a basis for listing a species 

as endangered. 

The anadromous form of the fish Oncorhynchus mykiss is thought to have undergone a significant range contraction at the southern extent of its 

freshwater range, although the phenomenon is 

neither well characterized nor well understood 

(Boughton 2005). Climate may have played a 

role—since the ending of the “Little Ice Age” in 

the 19th Century, the southern California climate 

has become warmer, which would be expected to 

drive a range contraction (Millar and Woolfenden 

1999; Haston and Michaelsen 1997; Scuderi 1993). 

However, the climate has also become wetter and 

less erratic, both of which would be predicted to 

allow the fish to expand their range; the first by 

making more kilometers of stream suitable to the 

fish, and the latter by providing for more stable 

metapopulation dynamics. 

The contraction could also be the result of 

other forces. Population decline is a widespread 

phenomenon in steelhead, occurring throughout 

the entire west coast of the USA (Busby et al. 1996), 

and the range contraction may simply be its artifact (i.e., population extirpation may not be any 

more common in the far south than elsewhere). 

Also, the southern limit of anadromous O. mykiss

happens to coincide with the Los Angeles – San 

Diego metropolitan area, where water diversion 

and flood control practices have profoundly transformed the region’s hydrology, with many negative impacts on the region’s freshwater fishes 

(Moyle 1995). Which combination of the above 

mechanisms might explain the range contraction is 

not clear, nor is the precise nature of the range 

contraction itself. 

In this paper we analyze the pattern of population extirpation for anadromous O. mykiss (hereafter, “steelhead”) in southern California (Figure 

1). First, we systematically assess the occurrence of 

steelhead in each basin in which it historically occurred. Occurrence was determined using a combination of basin-scale reconnaissance and siteFigure 1. The study area. 
2 anadromous O. mykiss 

level spot checks (snorkel surveys). To estimate 

detection power during the spot checks, we assumed a simple model of detection based on a 

Poisson process. 

Second, we test the hypothesis that the probability of basin-level extirpation is related to latitude; and then test whether two types of extirpations—those associated with barriers and those 

not—are correlated with certain environmental 

variables, namely basin size, mean annual air 

temperature for the period 1961 – 1990, and percent of the basin that has undergone urban or agricultural development. 

Methods 

Study system.—Steelhead in the study area inhabit a diverse set of watersheds, from redwood 

forests in the north to systems of chaparral and 

sycamore canyons in the south. Despite this diversity a number of ecological features unite the 

study area and differentiate it from steelhead habitat further north. Rainfall is low, episodic, and 

quite patchy in space; there is also high interannual variability in total precipitation. Streamflow 

can vary seasonally over five orders of magnitude, 

largely as a result of this patchiness (Figure 2). 

Summers are hot and dry. Many stream networks 

discharge so little water in the summer that the 

surface of the streambed dries up for long 

stretches (all discharge is groundwater or hyporheic). Perennial flow may be maintained in upper 

tributaries, particularly in mountainous areas 

where shallow bedrock pushes groundwater to 

the surface. 

At the ocean, the mouths of most streams become “barred over” during the summer by wave 

action building up beach sand; migration connectivity to the ocean is lost until the first large storm 

of the year washes out the sandbar (typically November or December). In the meantime the oversummering juveniles must survive in cool-water 

refugia within what are essentially small desert 

streams, or in lagoons isolated from the ocean by 

sandbars and from the stream system by dry 

mainstems. Southern California populations are 

thought to be very sensitive to variation in summer temperatures and annual rainfall. 

Under the U.S. Endangered Species Act, the 

anadromous form of O. mykiss is currently listed 

Figure 2. Hydrographs for a river in typical coastal steelhead habitat in Oregon (Alsea River), a system just north of the study area (San Lorenzo River), and a steelhead enclave in the southern part 

of the study area (Sespe Creek). Watershed area is given in parentheses. The time period includes 

the end of the California drought in early 1990s. 
 Contraction of the southern range limit 3 

 

as threatened or endangered in many parts of the 

western USA (Busby et al. 1996), and southern 

California is no exception. The actual entities 

listed under the act are “Evolutionarily Significant 

Units” of the species rather than the species itself 

(abbreviation: ESU; Busby et al. 1996). Two ESUs 

occur within the study area—the South-Central 

California Coast ESU and the Southern California 

ESU. The southern-most population in the study 

area is believed to be the southern geographic 

limit of anadromous steelhead as a whole. A scattering of isolated O. mykiss populations occur further south in Baja California and mainland Mexico 

(Nielsen et al. 1998) but do not appear to exhibit 

anadromy (e.g., Ruiz-Campos and Pister 1995). 

Most are considered to be distinct subspecies of 

freshwater trout by Nielsen et al. (1998). 

Over the course of the 20th century, the human 

population of southern California has rapidly expanded, in the process constructing an extensive 

array of dams, flood-control channels, and road 

networks, all of which involve the blocking or rerouting of stream channels as well as the reconfiguring of the natural hydrology of the region. 

Interestingly, much of this activity is focused 

on the lower mainstems (or lower parts of principal tributaries) of the stream systems. With certain 

exceptions (e.g., the Big Sur Coast), these lower 

mainstems tend to occur in wide alluvial valleys 

with extensive urban or agricultural development, 

exemplified by the Los Angeles basin and the 

Salinas Valley respectively. It seems reasonable to 

believe that such habitats originally served steelhead primarily as migration corridors, and that 

most spawning occurred higher in the basin in 

small to medium mountain streams. Despite the 

occurrence of a rather large human population, 

many of these mountain streams currently lie 

within the boundaries of National Forests and 

military bases, and there are numerous anecdotal 

accounts of extant non-anadromous O. mykiss

populations in portions of the basin currently isolated above anthropogenic barriers (Good et al. 

2005). 

Biological Assumptions.— Steelhead are sometimes observed to produce non-anadromous progeny and vice-versa (Zimmerman and Reeves 

2000), and in the juvenile stage the two forms are 

currently considered to be indistinguishable. For 

the purpose of this study we assumed that juvenile O. mykiss were members of populations capable of exhibiting anadromy (i.e., steelhead) if they 

were observed in stream reaches with migrationconnectivity to the ocean. Our concept of connectivity included stream networks that are seasonally dry in some sections, but exhibit discharge 

during the rainy season when the fish migrate. 

The grain of the occurrence data (the finestscale unit for which occurrence was estimated; 

Kotliar and Wiens 1990) was set as the individual 

coastal basin, defined as the entire catchment area 

for a stream at the point that it enters the ocean. 

The two largest coastal basins—the Salinas River 

system and the Pajaro River system—were divided into subbasins, defined by the catchment 

areas of each major tributary to the main stem. The 

extent of the study was the coastal region between 

the Pajaro River basin (inclusive) in the north and 

the U. S. border with Mexico in the south (Figure 

1). 

Historic Distribution.—The basins in the study 

area were classified as historic steelhead basins, or 

as having no evidence of historic occurrence, 

based on one public report (Stoecker et al. 2002) 

and three unpublished manuscripts obtained by 

the Southwest Fisheries Science Center, Santa 

Cruz Lab (Titus et al. 2003, Sleeper 2002, and 

Franklin 1999). Titus et al. (2003) thoroughly reviewed historic data on steelhead occurrence 

throughout the entire study area; the others collected “new” historical data in the form of interviews with elderly fisherman, etc., each for a specific region within the study area (Stoecker et al. 

2002: s. Santa Barbara County; Sleeper 2002: Orange and n. San Diego Counties; Franklin 1999: 

upper Salinas Basin). Another significant effort at 

historical research, for Ventura and Santa Barbara 

Counties, was unavailable to us (E. Henke, personal communication). Data indicating sporadic 

strays rather than evidence of spawning or rearing 

were not considered sufficient to establish historic 

presence of the species in a basin. Historic distribution as determined from these sources is depicted in Figures 3 and 4. 

 
4 anadromous O. mykiss 

Figure 3. The historic distribution of spawning and rearing basins for steelhead in the south-central 

California coast. The Pajaro and Salinas systems were disaggregated into sub-basins (for clarity, 

their mainstems are not depicted as habitat). Historic distribution is based on accounts from Titus et

al. (2003) and Franklin et al. (1999). 
 Contraction of the southern range limit 5 

 

Figure 4. The historic distribution of spawning and rearing basins for steelhead in southern California. Note that the grain of the occurrence data (i.e. the minimum geographic unit depicted) is the coastal basin: Shading of entire basins implies only that steelhead occurred somewhere, not necessarily everywhere, in a basin. Historic distribution is based on accounts from Titus et al. (2003), Stoecker et al. (2002) and 
6 anadromous O. mykiss 

Determining Occurrence.—Error rates in presence/absence data should tend to be asymmetric—

if a species is detected somewhere in a basin it will 

be known to be present, but if it is not detected it 

may either be absent or present but overlooked 

(an error). This implies that the presence of the 

species is consistent with both a positive and a 

negative observation, whereas absence is consistent only with a negative observation. We thus 

considered it parsimonious to treat “presence” as 

a null hypothesis, to be rejected by one of the following observations: 

Dry Stream: Drainages that were dry up to the 

first barrier to anadromy during summer 2002 

or 2003. This determination was made during 

reconnaissance by experienced field crews. 

Barrier Exclusion: Drainages with anthropogenic 

migration barriers between the ocean and any 

potential spawning habitat upstream. Our concept of potential spawning habitat was liberal: 

everything except concrete-lined channels, estuaries, marshes, and channels not exhibiting 

pool-run-riffle structure. We also placed in this 

category any stream fitting the definition of 

“Dry Stream,” but in which the limit to anadromy was anthropogenic and appeared to 

have spawning and rearing habitat above it. 

Spot Checks: If neither of the above criteria could be 

used to reject presence, we performed a spot 

check of occurrence using snorkel surveys. To 

maximize our power to detect fish, we used the 

method of “best-occurring habitat,” which involves a subjective judgement of relative habitat quality. A reach was judged to be “fishy” 

(i.e. relatively high quality) if it met the following criteria: i) moderate to high stream gradient; ii) pool-run-riffle structure; iii) high vegetative cover; and iv) low turbidity. The criteria 

were judged by experienced field crew during 

a period of reconnaissance by car. 

To conduct the spot check itself, a crew member conducted a snorkel survey (Dolloff et al. 1993) 

of all pools and runs, omitting riffles due to an 

assumed low detection-probability, and working 

from down- to up-stream. The survey stopped 

when either one or more steelhead were detected 

[scored as “spot check (observed)”], or when 100m 

total linear length of pools and runs had been surveyed without detection [scored as “spot check 

(not observed)”]. In either case, the linear distance 

of pools and runs surveyed (“distance-todetection”) was recorded as a measure of effort. 

 When we encountered a non-historic basin 

that looked fishy and had time to survey it, we did 

so. Occasionally the null hypothesis could not be 

tested in historic basins, either because landowners would not grant us permission or because the 

stream posed some hazard such as pollution or 

overhead bombing. We classified these basins as 

inaccessable. In some basins, particularly in southern Santa Barbara County, one of us (M.S.) had 

previously made numerous observations of occurrence during 2000-2002; these were scored as “spot 

check (observed)” if O. mykiss had been unambiguously detected; otherwise the basin was submitted to a formal spot check. In several cases steelhead had been documented within the past 3 yr 

(2000 – 2002) by practicing fisheries biologists; we 

also scored these basins as “spot check (observed)” without applying the formal method. 

Estimating detection error.—To estimate detection error at the site-level, the effort data (distances-to-detection for each spot check) were calibrated by a statistical model that assumed detection of fish to be a Poisson process (so to speak). 

The model is based on the idea that the longer the 

distance surveyed without detecting a fish, the 

more likely the species is truly absent; or to put it 

more precisely (see Appendix A), the complete 

absence of fish in 100m of habitat allowed us to 

reject the hypothesis of 100 or more juveniles per 

km of habitat at p = 0.01 and an assumed per-fish 

detection probability of 0.5. This last assumption is 

slightly conservative with respect to demonstrating absence; measured detection rates in snorkel 

surveys are about 0.6 (comparison with electrofishing counts in Ten-mile Creek, California; T. 

Williams and B. Spence, unpublished data). 

 The survey design has an important limit: if 

juvenile populations commonly occur at densities 

less than 100 per km in the best-occurring habitat, 

they would frequently go undetected. In fact, our 
 Contraction of the southern range limit 7 

 

results (to be described further on) suggest that 

nearly all populations have much higher densities. 

 We also pursued two strategies for assessing 

detection error at the level of entire basins. First, in 

a subset of basins we replicated spot checks by 

performing them at two or more sites within a 

basin. Error was estimated by the probability of 

disagreement between replicated spot checks. Second, in five subbasins of the Salinas, we compared 

the “best-occurring habitat” method with a concurrent general fish survey performed by a separate field team. The general method was designed 

to characterize the typical fish community of each 

sub-basin, and did not have a specific focus on 

identifying O. mykiss habitat (potentially quite 

patchy). We predicted that if the “best-occurring” 

strategy was effective, it should detect occurrence 

of O. mykiss for basins in which the general survey 

did not; but not the reverse. 

In the general survey, one or more 500m 

reaches typical of the basin were selected and then 

surveyed. A snorkel-survey was performed if 

maximum depth was > 25cm; otherwise bank-side 

observations were made using either polarized 

glasses or Aqua-Vu underwater viewing equipment (Nature Vision Inc.). Riffles were included in 

the survey; two divers sometimes performed the 

survey in wide streams; and all fish species present were identified and counted. 

Hypothesis tests.—Logistic regression was used 

to test a series of hypotheses against the occurrence data. To test hypotheses about the range 

contraction, the data were coded as “extant” vs 

“extirpation,” the latter defined as an observed 

absence in a historical steelhead basin. For preliminary analysis of latitudinal clines in extirpation, basins were grouped into geographic positions according to the county containing the centroid of each (Table 1). We then examined underlying reasons for extirpations. Specifically, we 

tested the statistical significance of the following 

predictors: mean annual air temperature by basin; 

basin size; and the percent of basin that has been 

developed by the human population (Table 1). The 

relationship with basin size (area) is a general biogeographic prediction; the relationship with temperature would be expected if mean annual temperature is a limiting factor in the study area; and 

the relationship with development would be expected if direct human impacts were the primary 

cause of the range contraction. 

Basin areas (hectares) were estimated using 

ArcMap software and then log-transformed. Mean 

annual temperatures for each basin were taken as 

the mean PRISM temperature across each basin, 

where PRISM refers to interpolated climate data 

computed as in Daly et al. (1994) (obtained from 

the Climate Source, Corvallis Oregon; mean °C for 

the period 1961 - 1990). Percent development was 

estimated using National Land Cover Data 

(NLCD), California-South section, version 04-28-

2000, available from the USGS EROS data center. 

NLCD categories were reclassified as a boolean 

coverage, in which the NLCD categories “Developed,” “Non-natural Woody,” and “Herbaceous 

Planted/Cultivated” were reclassified 1, and all 

other categories as 0. 

Regressions were done separately for the “barrier exclusions” and “other extirpations,” i.e., extirpations left over after barrier exclusions were 

removed from the dataset. The two regressions 

were made conditionally independent by coding 

the data thus: 

Significance tests were Bonferroni-corrected for 

multiple comparisons. 

TABLE 1. INDEPENDENT VARIABLES USED IN 

LOGISTIC REGRESSIONS. 

Geographic position codes 

1 Santa Cruz/Monterey Counties 

2 San Luis Obispo County 

3 Santa Barbary County 

4 Ventura/Los Angeles Counties 

5 Orange/San Diego Counties 

 

Environmental predictors 

Mean annual air temperature (°C, 1961 – 1990) 

Basin area (ln[Ha]) 

Percent Developed (%, from NLCD database) 
8 anadromous O. mykiss 

Results 

Overall we examined 86 of 92 historic steelhead basins (the remaining 6 were inaccessible), 

and 55 of 101 non-historic basins. These numbers 

include 22 subbasins of the Salinas and Pajaro, 11 

of which were historic steelhead basins. 

Occurrence.—Assuming negligible detection 

failure, the overall probability of occurrence in 

historic basins was between 0.59 and 0.65 (n = 92); 

the uncertainty is due to the 6 basins not assessed. 

For the south-central California ESU, the probability of occurrence was 0.80 – 0.87 (n = 45); for the 

southern California ESU it was 0.38 – 0.45 (n = 47). 

Detection error.—Was detection failure truly 

negligible, as assumed by the above calculations? 

To answer this question, we made 17 revisits to 

basins already spot-checked. In all 17 revisits the 

score (detected; not detected) was the same as 

that of the previous spot check, for an error probability of 0.00 (95% c.i.: [0.00, 0.162] by the binomial distribution). Additionally, we compared the 

best-occurring habitat method with a general survey method in 5 sub-basins of the Salinas. The 

best-occurring habitat method detected O. mykiss

in 3 sub-basins in which it was not detected by the 

general method, and agreed with the general 

method in the other two sub-basins (Table 2). 

When steelhead were observed during a spot 

check, they were usually observed within 30m of 

the start point of the snorkel survey (32 of 36 

cases), suggesting that surveying 100m of pools 

and runs was sufficient effort to detect the species 

with fairly high confidence. This assertion can be 

quantified by fitting a probability-density function 

to the dataset (the distances-to-detection), and estimating error rate as the area under the righthand tail of the distribution, 

probability of detection failure ∫

∞

=

m

pdf x dx

100

( ) . 

This is a simple failure-time model (Smith 2002). 

In words, the above quantity is the probability of 

not detecting the species in the first 100m, conditional on it being present and detectible under infinite effort. A truncated log-normal distribution 

(Johnson et al. 1994) was found to fit the data (χ2 = 

1.914, df = 2, p = 0.384); the estimated probability of 

detection failure was 0.0175. Overall this analysis 

suggests that few extant populations tend to be 

sparser than 1 observable fish per 100m of habitat, 

and that our spot-check method had a decently 

low error rate. This implies, via the statistical 

model described in Appendix A, that most juvenile populations have densities well above 100 fish 

per km in the best-occurring (or “fishy”) habitat. 

TABLE 2. COMPARISON OF O.MYKISS DETECTION USING TWO SURVEY METHODS. 

 General fish survey method1 Best-occurring habitat method2

Sub-watershed Historical 

occurrence 

Current 

occurrence 

Number of 

detections 

Current 

occurrence 

Number of 

detections 

Arroyo Seco Yes Present 4 of 7 sites Present 1 of 1 sites3

Paso Robles Yes Absent 0 of 1 sites Present 2 of 3 sites 

Atascadero Yes Absent 0 of 2 sites Present 1 of 1 sites 

Santa Margarita Yes Absent 0 of 1 sites Present 1 of 1 sites 

Chalone No Absent 0 of 2 sites Absent 0 of 1 sites 

1 General method did not focus on best-occurring O. mykiss habitat, but rather stream habitat typical of the sub-basin, for the purpose of characterizing the most common fish species in the sub-watershed. 

2 Best-occurring method focused on detection of O. mykiss only. 

3 Data collected in 2001 under a different detection protocol, but similar site-selection protocol. 
 Contraction of the southern range limit 9 

 

Geographic patterns.—The historic steelhead 

basins exhibited a latitudinal trend in extirpation 

rate (null hypothesis: p = 0.00008; Table 3A), 

clearly depicted in Figure 5. Most of the extirpations were associated with migration barriers: Of 

those historic basins that were assessed, 26% had 

barrier exclusions, 1% were dry, and 11% were 

observed to be vacant during the spot check (n = 

85). The rest were occupied. 

Historic occurrence of steelhead was related to 

both basin area and mean annual temperature 

(Table 3B); however current occurrence of steelhead was no longer statistically associated with 

basin area (p = 0.083; Table 3C). Inspection of the 

confidence limits for this predictor in Table 3B and 

3C suggests that the relationship of basin area and 

species occurrence is now quite different from 

what it was historically. In contrast, the confidence 

limits for the historic and current effect of mean 

annual temperature are quite similar. 

The extirpations classified as barrierexclusions tended to be associated with percent 

development of the basin (Table 3D); there was no 

significant association with basin area, nor mean 

annual temperature. In contrast, the other “other” 

extirpations tended to be associated with warmer 

mean annual temperatures but not percent development (Table 3E). 

New records of occurrence.—Five new records of 

occurrence were made in the 56 non-historical basins surveyed. One of these was Gabilan Creek, a 

tributary to the old Salinas River Channel. Gabilan 

Creek is the only subbasin on the arid east slope of 

the Salinas Valley known to harbor juvenile O. 

mykiss. The other four new records were scattered 

along the Big Sur Coast and Morro Bay region. 

They occur amidst other basins of similar size and 

environmental character that also harbor steelhead, and in addition have been said to harbor 

resident populations of O. mykiss above impassable barriers (Titus et al. 2003) 

Figure 5. Extirpation north to south in the study area. Data are for all historical steelhead basins except six that were inaccessible (n = 85). Numbers in parentheses are the 

number of basins for each geographic position. 
10 anadromous O. mykiss 

TABLE 3. HYPOTHESIS TESTS USING LOGISTIC REGRESSION. 

 Predictor Estimate (95% c.l.) t(df) p Odds Ratio* 

A. Is there a latitudinal cline in extirpation rate?

Indep. var.: All basins, classified as extirpations vs. not (χ2 = 20.73; df = 1; p = 0.00001). 

 Intercept -2.95 (-4.32, -1.60) -4.33 (84) 0.00002 

 Geographic Position 0.904 (0.45, 1.36) 3.95 (84) 0.00008 2.47 (1.57, 3.89) 

B. Is the historic occurrence of steelhead in a basin related to basin size or to climate?

Indep. var.: All basins, classifed as historic steelhead vs. not historic (χ2 = 33.29; df = 2; p = 0.00002) ‡. 

 Intercept 2.74 (-1.26, 6.74) 1.35 (193) 0.18 

 Mean Annual Temp. -0.41 (-0.67, -0.15) 3.17 (193) 0.0018 0.66 (0.51, 0.86) 

 Ln(Basin Area) 0.42 (0.27, 0.61) 4.49 (193) 0.0001 1.53 (1.27, 1.84) 

C. Is the current occurrence of steelhead in a basin related to basin size or to climate?

Indep. var.: All basins, classifed as currently occupied vs. not (χ2 = 19.90; df = 2; p = 0.0010).‡

 Intercept 8.28 (3.03, 13.54) 3.12 (139) 0.0022 

 Mean Annual Temp. -0.67 (-1.02, -0.32) 3.77 (139) 0.0002 0.51 (0.36, 0.73) 

 Ln(Basin Area) 0.16 (-0.02, 0.35) 1.75 (139) 0.083 

D. Are cases of barrier exclusion correlated with certain environmental predictors? 

Indep. var.: All basins, classified as barrier-exclusions vs. not (χ2 = 12.57; df = 3; p = 0.012).‡

 Intercept -9.44 (-18.9, 0.037) 1.98 (83) 0.051 

 Mean Annual Temp. 0.52 (-0.075, 1.11) 1.74 (83) 0.086 

 Ln(Basin Area) -0.04 (-0.35, 0.28) 0.24 (83) 0.81 

 Percent Developed 5.24 (0.96, 9.53) 2.43 (83) 0.017 1.69 (1.10, 2.59) 

E. Are other types of extirpations correlated with certain environmental predictors?

Indep. var.: Basins without barrier-exclusion, classified as extirpated vs. not (χ2 = 10.38; df = 3; p = 0.03).‡

 Intercept -18.4 (-29.4, -7.33) -3.32 (64) 0.0015 

 Mean Annual Temp. 1.00 (0.33, 1.67) 2.99 (64) 0.0040 2.72 (1.40, 5.31) 

 Ln(Basin Area) 0.20 (-0.22, 0.62) 0.94 (64) 0.35 

 Percent Developed 0.33 (-6.6, 7.2) 0.09 (64) 0.93 

*

 Odds ratio is for a unit change in the predictor (i.e. 1 ̊C increase in mean annual temp; approximate tripling of basin size; or 

10 percentage points of urban/agricultural development). 95% confidence intervals in parentheses 

‡ p-values for model significance are Bonferroni-corrected for multiple comparisons. 

 
 Contraction of the southern range limit 11 

 

Discussion 

Detection failure.—We classified a population 

as “extirpated” if we failed to observe fish during 

a spot-check; naturally, this rapid-assessment 

technique may miss some extant populations—a 

so-called detection failure. Yet our efforts at 

measuring error suggested that such detection 

failures were rare: about 1.75% at the reach level, 

and about 0% at the basin level given an accurate 

detection at the reach level (however, the latter 

had a confidence interval that included 16%). 

These estimates suggest that the rapid-assessment 

technique had a reasonably low error rate, and 

most of the apparent extirpations were true extirpations. Even so, a more intensive study might 

turn up additional extant populations, either because error rates were higher than thought, or because some of the vacant basins subsequently got 

colonized. 

Similarly, some of the “barrier exclusions” 

that we report may turn out to be partial impediments to steelhead migration, rather than outright 

barriers. Since many of these barriers have extant 

populations of O. mykiss above them (see below), 

these populations might have to be reclassified 

from freshwater-resident to possibly anadromous. 

Unfortunately, the passability of small instream 

barriers by adult steelhead appears to be an intricate and poorly understood subject. Opinion varies widely about the abilities of steel-head with 

respect to barriers/impediments. 

Patterns of extirpation.—Given that the rate of 

detection failure appears to be low, the data indicate that a large range contraction of anadromous 

O. mykiss has occurred. In the study area, steelhead populations have apparently been extirpated 

from at least a third of the basins in which they are 

known to have occurred historically. The evidence 

suggests that most of these extirpations occurred 

in the south, and about two-thirds of them were 

associated with human-built barriers that block 

migratory access. 

 On the other side of the ledger, occupancy was 

surprisingly high in the Big Sur – San Luis Obispo 

region, even in quite small coastal basins. Occurrence was in fact discovered in four small basins 

that had no previous record of occurrence. The 

high occupancy rate of steelhead populations in 

the tiny basins is surprising—not because the 

habitat seems unsuitable, but because the stream 

networks seem too small to support persistent 

populations. Typically one would expect that 

small populations periodically go extinct naturally, leaving a period of natural extirpation before 

being recolonized by fish from nearby areas. If this 

were the case in the Big Sur, there should have 

been some fraction of basins vacant at any given 

time, including the time of our survey. Yet there 

was really only one site that was both vacant and 

accessible: Cayucos Creek. This suggests that Big 

Sur populations are quite resilient to extinction 

despite being small. Perhaps there is much interbasin movement of fish, effectively linking the 

various coastal basins into one large population. 

As for basins suffering from barrier-exclusion, 

some points are worth noting. Strictly speaking, 

the existence of a barrier exclusion does not indicate that the barrier was actually responsible for 

the extirpation—the population could have gone 

extinct even if the barrier had never been built. 

However, if this were the case the portion of the 

population trapped above the dam should also 

have gone extinct. In fact in 15 cases the opposite 

is true: the resident form of the species continues 

to occur above the barrier (Table 4; data from 

Good et al 2005 and Stoecker et al. 2002). These 

populations cannot be reached by anadromous 

adults migrating upstream, yet indicate that at 

TABLE 4. OCCURRENCE OF RESIDENT, RESIDUALIZED, OR 

STOCKED O. MYKISS ABOVE ANTHROPOGENIC BARRIERS, 

FOR BASINS IN WHICH O. MYKISS HAS BEEN EXTIRPATED 

BELOW THE BARRIERS. 

Observations of O. 

mykiss above barriers 

Proportion Type of 

extirpation below 

barriers Recently 

observed* 

Not 

observed 

Barrier exclusions 15 7 0.68 

Other extirpations 2 8 0.2 

* observed sometime within the past 10 yr, cf. Good et al. (2005) or 

Stoecker et al. (2002). See appendix C for names of basins. 
12 anadromous O. mykiss 

least this number of basins continues to possess 

suitable O. mykiss habitat. If migrationconnectivity were restored for the 15 residualized 

populations noted in Table 4, and anadromous 

fish began contributing offspring to the population, the total number of steelhead extirpations in 

the study area would be halved. The overall rate 

of extirpation would then be no greater than what 

has already been inferred from our data on 

“other” extirpations: about 16%. Note that the 

figure could be larger, because the “not observed” 

category in Table 4 probably includes some undetected residualized populations. 

The logistic regressions suggest extirpations of 

two types: those associated with temperature, and 

those associated with barriers arising from urban 

and agricultural development. In particular, 

1) Occurrence of steelhead at the basin level is no 

longer related to basin size, though it apparently once was. The original pattern was an example of the species-area relationship, one of 

the most general ecological patterns observed 

in nature. It probably no longer holds because 

the barrier exclusions have made the effective 

size of many stream networks much smaller 

than the actual size. The amount of accessible 

habitat is no longer closely related to basin size. 

2) Barrier exclusions tended to be associated with 

highly-developed basins, whereas the other extirpations had no relationship with development. At least half of the barrier-exclusions still 

have extant O. mykiss populations in streams 

above the barrier, indicating suitable conditions 

for the non-anadromous form of the species. 

3) The other extirpations tended to be associated 

with warmer basins, whereas the barrier exclusions were not. There is no a-priori reason to 

expect barriers to be built in warmer basins. In 

contrast, there is an expectation that at the 

southern extent of the range, the populations in 

the warmest basins would be most vulnerable 

both to short-term climate fluctuations and 

long-term trends. 

A role for climate is not surprising, since climate variability at the scale of decades is thought 

to strongly influence marine productivity of 

salmon across the northern Pacific (e.g., Hare and 

Francis 1995, Hare 1996, Mantua et al. 1997) as well 

as having local effects on the hydrology and sediment load of southern California rivers (Inman 

and Jenkins 1999). Over the long term, there has 

been a warming trend in California and elsewhere 

following the end of the “Little Ice Age” in the 19th

century (Mann et al. 1998, Millar and Woolfenden 

1999), and shifts in faunal abundance and distribution have been attributed to long-term climate 

trends in California for both terrestrial and marine 

organisms (Parmesan 1996, Barry et al. 1995). 

On the other hand, in the twentieth century 

southern California has also seen a trend toward 

wetter conditions (Inman and Jenkins 1999) that 

would presumably benefit the fish. Indeed, during 

the 1990s steelhead recolonizations were observed 

to reverse extirpations in two basins, currently the 

two southernmost populations of steelhead in the 

study area (T. Hovey, pers. comm.; Dagit and 

Webb 2002). To complicate the picture even more, 

certain climate models suggest that if the current 

warming trend continues in California over the 

next century, it will have particularly strong effects in the south in terms of temperature and 

rainfall (Snyder et al. 2002, Snyder et al. 2003). 

 Despite the importance of climate, extirpations 

associated with barriers were twice as common as 

extirpations associated with temperature. The 

strong association between range contraction of an 

aquatic species and the regional development of 

water resources and flood control is consistent 

with Moyle’s (1995) vision of the threats facing 

freshwater fish in southern California, and Frissell’s (1993) analysis of threats to freshwater fishes 

throughout the west coast of the USA. Our study 

is the first to conduct a basin-by-basin accounting 

of O. mykiss in the south, and unambiguously 

demonstrates a large-scale range contraction and a 

strong association of barriers with that range contraction. Unlike the myriad impacts of climate 

change on natural systems (Parmesan and Yohe 

2003), and the numerous negative effects of urbanization on native species, the major anthropogenic limit on southern steelhead occurrence at the 
 Contraction of the southern range limit 13 

 

basin scale appears to be quite specific: anthropogenic barriers are preventing migration to suitable 

habitat. 

Acknowledgements 

 A regional-scale study such as ours is impossible without the help of a large set of people. 

Charlene Bergeron, of NOAA Fisheries SW Fisheries Science Center, helped organize stream lists. 

Brycen Swart, Selena Gell, Steve Gough, Morgan 

Kilgour, Holly Kindsvater and Jon Reum also contributed to the fieldwork. Tommy Williams, Brian 

Spence, Pete Adams, and Michael Mohr, all of the 

Center, provided extensive logistical support. 

Craig Wingert of the NOAA Fisheries SW Regional Office facilitated our study and Mark 

Capelli of the same office provided help on the 

ground and extensive background information. 

The following fisheries and wildlife biologists 

shared observations of steelhead occurrence or 

stream condition with us: K. Cooper with the Los 

Padres National Forest; D. Dettman with the Monterey Peninsula Water Management District; C. 

Garza, T. Williams, B. Spence, and T. Laidig, with 

the NOAA Fisheries SW Fisheries Science Center, 

Santa Cruz; M. McEachron with the United Water 

District, Ventura County; C. White with the Camp 

Pendleton Marine Base; and C. Zimmerman with 

the USGS Alaska Science Center. 

We thank the following individuals for aiding 

us in obtaining right-of-entry to properties, or for 

providing information about the properties under 

their care: Pete and Sandy Desimone with the 

Audubon Starr Ranch Preserve; Lee Otter and 

Sharif Traylor with the California Coastal Commission; Dave Highland and Mauricio Cardenas 

with the California Department of Fish and Game; 

Art Fong, Ken Gray, Suzanne Goode, Vince 

Cicero, Gary Hughey, Roland Jacobs and Chuck 

Lowe with California State Parks; Jim Edmondson 

with California Trout; Bob Gresens with the 

Cambria Community Services District; Mike 

Reeves with the Gabilan Ranch; Dr. Chris White 

with the Camp Pendleton Marine Corps Base; Sister Francisca with the Carmelite Monastery; James 

Studarus with the Conception Coast Project; Carl 

W. Harry with the Department of General Services, San Diego County; Lee Shick with the Department of Public Works, San Diego County; Deb 

Lelevier with the Escondido Creek Conservancy; 

Tim Miller with Harbors, Parks and Beaches, 

County of Orange; Donna Toth, Jamie Uyehara, 

Chris Clervi, Kevin Cooper, Jeff Kwasny, Lynne 

Barnes, and John Brack with the Los Padres National Forest; Gina Brown with the Los Penasquitos Preserve, San Diego County; Dave Dettman 

with the Monterey Peninsula Water Management 

District; Harry Ehrlich and Todd Nornith with the 

Olivenhain Municipal Water District; Cory Linder 

with the Open Space Division, San Diego County 

Parks and Recreation; Sally Krenn with the PG&E 

Diablo Canyon Power Plant; Rosi Dagit with the 

Resource Conservation District of the Santa 

Monica Mountains; Mark Webb with San Diego 

County Parks; Susan Shanks and Paul Edelman 

with the Santa Monica Mountains Conservancy; 

Gary Busteed with the Santa Monica Mountains 

National Recreation Area, National Park Service; 

Kurt Kurner and Paul DeLorenzo with Stocker 

Resources; Chris Zimmerman with the United 

States Geological Survey; Murray McEachron with 

the United Water District; Isobel Kay with the 

University of California Reserve System; Nancy 

Read with Vandenberg Air Force Base; and Tom 

Malone with Woods Canyon/Laguna Niguel Park, 

County of Orange. 

Michael Hazzard with the Clean Aliso Creek 

Association and David Pritchett with Fishtap introduced us to the Aliso Creek basin and the Santa 

Monica Mountains, respectively. Appreciation to 

Gail Prothro for providing information about Orange County. Marc Shores prepared for us some 

highly useful aerial photos of properties owned by 

the Santa Monica Mountains Conservancy. 

We would like to thank many private landowners anonymously for their kindness and interest in our efforts. 

Literature Cited 

Alley, D.W. 2001. Trends in juvenile steelhead production in 1994-2000 for Santa Rosa Creek, San 

Luis Obispo County, California, with habitat 
14 anadromous O. mykiss 

analysis and an index of adult returns. Don Alley and Associates, Brookdale, California. 

Anonymous. 2000. Lower Santa Ynez River Fish 

Management Plan (Volume II Appendices). 

Barry, J.P., C.H. Baxter, R.D. Sagarin, and S.E. Gilman. 1995. Climate-related, long-term faunal 

changes in a California rocky intertidal community. Science 267: 672 – 675. 

Bayley, P.B. and J.T. Peterson. 2001. An approach 

to estimate probability of presence and richness 

of fish species. Trans. Am. Fish. Soc. 130: 620 – 

633. 

Boughton, D. A. 2005. Status review update, 

southern California steelhead ESU. In: Waples 

et al. Status review update for west coast salmonids. U.S. Dept. Commerce, NOAA Tech. 

Memo. [In Press]. 

Busby, P. J., T. C. Wainwright, G. J. Bryant, L. Lierheimer, R. S. Waples, F. W. Waknitz, and I. V. 

Lagomarsino. 1996. Status review of west coast 

steelhead from Washington, Idaho, Oregon, 

and California. U.S. Dept. Commerce, NOAA 

Tech. Memo. NMFS-NWFSC-27, 261p. 

Dagit, R. and C. Webb. 2002. Topanga Creek Watershed & Lagoon Restoration Feasibility 

Study. Resource Conservation District of the 

Santa Monica Mountains and Moffatt & Nichol 

Engineers. 

Daly, C., R.P. Neilson, and D.L. Phillips, 1994: A 

Statistical-Topographic Model for Mapping 

Climatological Precipitation over Mountanious 

Terrain. J. Appl. Meteor., 33, 140-158. 

Dolloff, C.A., D.G. Hankin, and Gordon H. 

Reeves. 1993. Basinwide estimation of habitat 

and fish populations in streams. Gen. Tech. 

Rep. SE-83. Ashville, NC: US Department of 

Agriculture, Forest Service, Southeastern Forest 

Experiment Station. 

Easterling, D.R., G.A. Meehl, C. Parmesan, S. 

Chagnon, T. Karl, L. Mearns. 2000. Climate extremes: observations, modeling, and impacts. 

Science 289: 2068-2074. 

Franklin, H. A. 1999. Steelhead and salmon migration in the Salinas River. Unpublished report, 

H. Franklin, 1040 South River Road, Paso 

Robles, CA 93446. 

Frissell, C. A. 1993. Topology of extinction and 

endangerment of native fishes in the Pacific 

Northwest and California (USA). Cons. Biol. 

7(2): 342 – 354. 

Good, T. P., R. S. Waples, and P. Adams. (eds.) 

2005. Updated status of Federally listed ESUs 

of West Coast salmon and steelhead. U.S. Dept. 

of Commerce, NOAA Tech. Memo., NMFSNWFSC-66. 

Hare, S.R. 1996. Low-frequency climate variabiliity 

and salmon production. Ph.D. thesis, University of Washington, Seattle. 306 pp. 

Hare, S.R. and R.C. Francis. 1995. Climate change 

and salmon production in the Northeast Pacific 

Ocean. Can. Spec. Publ. Fish. Aquat. Sci. 121: 

357 – 372. 

Haston, L. and J. Michaelsen. 1997. Spatial and 

temporal variability of southern California precipitation over the last 400 yr and relationships 

to atmospheric circulation patterns. Journal of 

Climate 10: 1836 – 1852. 

Houghton, J.T. 1997. Global warming: the complete briefing. Cambridge University Press, 

Cambridge. 

Inman, D.L. and S.A. Jenkins. 1999. Climate 

change and the episodicity of sediment flux of 

small California rivers. Journal of Geology 107: 

251 – 270. 

Johnson, N.L., S. Kotz, and N. Balakrishnan. 1994. 

Continuous univariate distributions, v. 1, 2nd 

edition. Wiley. 

Kareiva, P.M., J.G. Kingsolver, and R.B. Huey. 

1993. Biotic interactions and global change. 

Sinauer Associates, Sunderland. 

Kéry, M. 2002. Inferring the absence of a species—

a case study of snakes. J. Wildlife Manage. 

66(2): 330 – 338. 

Kotliar, N.B. and J.A. Wiens. 1990. Multiple scales 

of patchiness and patch structure: a hierarchical 

framework for the study of heterogeneity. 

Oikos. 59(2): 253-260. 

Lang, J.S., B.F. Oppenheim, and R.N. Knight. 1998. 

Southern steelhead Oncorhynchus mykiss habitat suitability survey of the Santa Margarita 

River, San Mateo and San Onofre Creeks on 

Marine Corps Base Camp Pendleton, California. US FWS, Arcata California. 

Mann, M.E., R.S. Bradley, and M.K. Hughes. 1998. 

Global-scale temperature patterns and climate 
 Contraction of the southern range limit 15 

 

forcing over the past six centuries. Nature 392: 

779 – 787. 

Mantua, N.J., S.R. Hare, Y. Ahang, J.M Wallace, 

and R.C. Francis. 1997. A Pacific interdecadal 

climate oscillation with impacts on salmon production. Bulletin of the Amercan Meteorological Society 78: 1069 – 1079. 

McArdle, B.H. 1990. When are rare species not 

there? Oikos 57: 276 – 277. 

McIntyre, S. and R. McKitrick. 2003. Corrections to 

the Mann et. al. (1998) proxy pata base and 

northern hemispheric average temperature series. Energy & Environment, 14(6): 751 – 771. 

Millar, C.I. and W.B. Woolfenden. 1999. The role 

of climate change in interpreting historical variability. Ecological Applications 9(4): 1207 – 

1216. 

Morro Group, Inc. 2002. Stenner Creek/Foothill 

Boulevard bridge replacement project, San Luis 

Obispo, CA; Natural environment study, California Red-legged Frog presence/absence survey, Wetland Assessment. Martin, Rivett and 

Olson, Inc. 

Moyle, P.B. 1995. Conservation of native freshwater fishes in the Mediterranean-type climate of 

California, USA: A review. Biological Conservation 72: 271 – 279. 

Musick, J.A. 1999. Criteria to define extinction risk 

in marine fishes. Fisheries 24(12): 6 – 14. 

Nielsen, J.L., M.C. Fountain, J.C. Favela, K. Cobble, B.L. Jensen. 1998. Oncorhynchus at the 

southern extent of their range: A study of 

mtDNA control-region sequence with special 

reference to an undescribed subspecies of O. 

mykiss from Mexico. Environmental Biology of 

Fishes. 51(1): 7-23. 

Parmesan, C. 1996. Climate and species range. Nature. 

Parmesan, C. and G. Yohe. 2003. A globally coherent fingerprint of climate change impacts 

across natural systems. Nature 421(6918): 37-42. 

Parmesan, C., N. Ryrholm, C. Stefanescu, J.K. Hill, 

C.D. Thomas, H. Descimon, B. Huntley, L. 

Kaila, J. Kullberg, T. Tammaru, W.J. Tennent, 

J.A. Thomas, and W. Martin. 1999. Poleward 

shifts in geographical ranges of butterfly species associated with regional warming. Nature 

399: 579-583. 

Payne, T.R. and Associates. 2001. The distribution 

and abundance of steelhead in tributaries to 

Morro Bay, California. Coastal San Luis Resource Conservation District. 

Ruiz-Campos, G. and E. P. Pister. 1995. Distribution, habitat, and current status of the San 

Pedro Martir rainbow trout, Oncorhynchus 

mykiss nelsoni (Evermann). Bulletin of the 

Southern Caliornia Academy of Sciences, 94(2): 

131 – 148. 

Scuderi, L.A. 1993. A 2000-year tree-ring record of 

annual temperatures in the Sierra-Nevada 

mountains. Science 259: 1433 – 1436. 

Sleeper. 2002. A historical survey of steelhead 

stockings and runs in Orange & northern San 

Diego Counties. Unpublished report. 

Smith, P.J. 2002. Analysis of failure and survival 

data. Texts in statistical science. Chapman & 

Hall/CRC, Boca Raton. 

Snyder, M.A., J.L. Bell, and L.C. Sloan. 2002. Climate responses to a doubling of atmospheric 

carbon dioxide for a climaticallly vulnerable 

region. Geophysical Research Letters, 29(11): 9-

1 – 9-4. 

Snyder, M.A., L.C. Sloan, N.S. Diffenbaugh, and 

J.L. Bell. 2003. Future cliamte change and upwelling in the California Current. Geophyscial 

Research Letters 30(15): 8-1 – 8-4. 

Stoecker, M.W. and CCP (Conception Coast Project). 2002. Steelhead assessment and recovery 

opportunities in Southern Santa Barbara 

County, California. Conception Coast Project, 

Santa Barbara. 

Swift, C.C., R. N. Fisher and M.L. Warburton. 

2000. Status and Distribution of fishes in the 

Santa Margarita River Drainage. San Diego, 

CA. The Nature Conservancy. 

Titus, R.G., D.C. Erman, and W.M. Snider. 2003. 

History and status of steelhead in California 

coastal drainages south of San Francisco Bay. 

Unpublished report. 

Zimmerman, C. E. and G. H. Reeves. 2000. Population structure of sympatric anadromous and 

nonanadromous Oncorhynchus mykiss: evidences from spawning surveys and otolith microchemistry. Can. J. Fish. Aquat. Sci. 57: 2152 – 

2162. 
16 anadromous O. mykiss 

Appendices 

Appendix A. Statistical model used in spot checks 

If w is the probability that a small stream segment contains an observable fish, then the probability of not observing fish in n independent 

identical segments is n (1− w) (McArdle 1990, 

Kéry 2002, Bayley and Peterson 2001). Assume the 

segments to be joined end-to-end in a single transect; subdividing each by an integer m gives the 

probability of not observing the species, 

nm

m

w (1− ) , 

in which individuals are randomly distributed 

among the new subdivided sites. At the limit m →

∞, the expression becomes e-wn, where e is the base 

of natural logarithms. Now, n is simply the length 

of the transect and w is the rate of detections per 

unit distance, or equivalently, the density of observable animals. This is a description of a Poisson 

process. 

An important conclusion immediately follows: 

Choose a random point on the transect and the 

distance Y to the next animal is exponentiallydistributed, satisfying the equation 

(1) wy

wy

e wy e S y Y y −

−

= > = = 0!

( ) ( ) Pr{ }

0

 

(see Smith 2002:20), where the notation for n has 

been changed to y for clarity. Smith (2002: 123) 

also describes the corresponding likelihood equation, 

(2) ∏ ∏ = =

=

R

j

D

i

i L w c f Y S y

1

max

1

( ) ( ) ( )

in which f(y) is a probability-density function associated with S(y) [ f(y) = −S ′(y)]. The D observations of the first product denote transects that terminated in species detections (distance-todetection for transect i is Yi). The R observations of 

the second product denote transects in which the 

species was not observed and the transect ended 

at a maximum length ymax. The value c is an arbitrary constant. If the species is not detected, equation (2) simplifies to 

 (3) ∏=

− = = =

R

j

R Rwy L w c S y cS y ce 1

max max max ( ) ( ) ( ) , 

which has the maximum-likelihood estimate 

wˆ = 0 [i.e., the maximum value of L(w) is at the 

lower bound w = 0]. For our purposes we are more 

interested in the uncertainty about this estimate, 

which can be obtained by deriving a cumulative 

density function (this is done by noting that the 

upper bound of w is infinity and solving the appropriate integral). The result is 

(4) max Pr( | ) ( ) w Ry

crit crit crit data w w CDF w e − > = = . 

To “establish” an absence one would reject this 

hypothesis at a specified p-value and critical density wcrit, the latter being set close enough to zero 

to be biologically satisfying. Substituting R = 1 and 

ymax = 0.10 km, and solving for a p-value of 0.01 

yields wcrit = 46 ≈ 50 fish per km. If one makes the 

conservative assumption that only about half of 

the fish are observable, then surveying 100m is 

sufficient to reject the hypothesis of 100 or more 

juveniles (= 50 ÷ 0.5) per km of stream habitat, at 

the level of p = 0.01. 
 Contraction of the southern range limit 17 

 

Appendix B. The occurrence data by basin. 

Historical 

Occurrence 

Current 

Occurrence 

Basin 

(north to south) Area (ha) Year (and external source for data) 

X X Pajaro River 337558 

 Principal subbasins 

X X Corralitos Creek 2001 (C. Garza et al. pers. comm.) 

X X Pescadero Creek 2003 

X X Uvas Creek 2003 

X Dry Llagas Creek 2003 

Absent San Benito River 2003 

X X Pacheco Creek 

 Elkhorn Slough 17526 

X X Salinas River 1137371 

 Principal subbasins 

 X Gabilan Creek 2000 

Dry El Toro Creek 2003 

Dry Chualar Creek 2003 

X X Arroyo Seco River 2002 

 Stonewall Creek 

Dry Chalome Creek 2003 

Dry San Lorenzo Creek 2003 

Dry Pancho Rico Creek 2003 

X Barrier San Antonio River 2003 

X Barrier Nacimiento River1 2003 

Dry Big Sandy Creek 2003 

Dry Estrella River 2003 

Dry San Marcos Creek 2003 

Dry Huerhuero Creek 2003 

X X Paso Robles Creek 2003 

X Extirpated Graves Creek 2003 

X X Atascadero Creek 2003 

X X Santa Margarita Creek 2003 

 Del Rey Creek 6965 

Barrier Seal Rock Creek 393 2002 

Absent Pescadero Canyon Creek 375 2002 

X X Carmel River 65822 2002 (D. Dettman, pers. comm.) 

X X San Jose Creek 3572 2002 

Barrier Gibson Creek 249 2002 

 X Malpaso Creek 881 2002 

Barrier Soberanes Creek 828 2002 

Absent Doud Creek 736 2002 

X X Garrapata Creek 2782 2002 

X X Rocky Creek 1359 2002 

X X Bixby Creek 2940 2002 

X X Little Sur River 10379 2002 

X X Big Sur River 15125 2001 (C. Garza et al. pers. comm.) 

Dry Sycamore Canyon 678 2002 

Barrier Grimes Canyon 213 2002 

X X Partington Creek 978 2002 

 Hot Springs Canyon 1111 

Barrier Lime Creek 208 2002 

 

1 Marginal habitat below Nacimiento dam was snorkel surveyed in 2003; no O. mykiss observed. 
18 anadromous O. mykiss 

Historical 

Occurrence 

Current 

Occurrence 

Basin 

(north to south) Area (ha) Year (and external source for data) 

X X Big Creek 5774 2001 (C. Garza et al. pers. comm.) 

 X Vicente Creek 920 2002 

X X Limekiln Creek 2201 2002 

Absent Kirk Creek 362 2002 

X X Mill Creek 1670 2002 

Absent Wild Cattle Creek 449 2002 

X X Prewitt Creek 1633 2002 

X X Plaskett Creek 512 2002 

X X Willow Creek - Montery 4214 2001 (T. Laidig, pers. comm.) 

X X Alder Creek 1085 2002 

 X Villa Creek - Montery 1140 2002 

Absent Soda Spring Creek 174 2002 

X X Salmon Creek 2188 2002 

X X San Carpoforo Creek 9229 2002 

 Arroyo de los Chinos 481 

X Inaccessible Arroyo de la Cruz 11075 2002 (Observed by M. Stoecker, 1998) 

 Oak Knoll Creek 1672 

X Inaccessible Little Pico Creek 1886 2002 

X Inaccessible Pico Creek 3496 2002 

X X San Simeon Creek 8498 2001 (C. Garza et al., pers. comm.) 

X X Santa Rosa Creek 12324 2001 (Alley (2001) 

X X Villa Creek - SLO 5186 2002 

X Extirpated Cayucos Creek 2834 2002 

Absent Little Cayucos Creek 565 2002 

X Barrier Old Creek 5324 2002 

Barrier Willow Creek - SLO 887 2002 

X X Toro Creek 4023 2002 

X X Morro Creek 6452 2002 

X X Chorro Creek 11947 2001 (Payne & Assoc. (2001)) 

 X Los Osos Creek 6096 2001 (Payne & Assoc. (2001)) 

X X Islay Creek 2452 2002 

X X Coon Creek 2099 2002 

X X Diablo Canyon 1400 2002 

Dry Little Irish Canyon 111 2002 

Dry Irish Canyon 911 2002 

X X San Luis Obispo Creek 21339 2002 (Morro Group, Inc. (2002)) 

Barrier Wild Cherry Canyon 400 2002 

Barrier Pecho Creek 432 2002 

Dry Rattlesnake Canyon 357 2002 

X X Pismo Creek 9706 2002 

X X Arroyo Grande Creek 37996 2003 (M. Kilgour, pers. comm.) 

X X Santa Maria River 481084 2002 (K. Cooper, pers. comm.) 

Dry Shuman Canyon 5978 2002 

Absent San Antonio Creek 45161 2002 

X X Santa Ynez River 232543 2000 (Anonymous (2000)) 

Absent Honda Creek 3058 2002 

X Extirpated Jalama Creek 6389 2002 

Inaccessible Wood Canyon 1026 2002 

Inaccessible Damsite Canyon 427 2002 

Inaccessible Canada del Cojo 824 2002 

Inaccessible Barranca Honda 634 2002 

Inaccessible Canada de la Llegua 202 2002 
 Contraction of the southern range limit 19 

 

Historical 

Occurrence 

Current 

Occurrence 

Basin 

(north to south) Area (ha) Year (and external source for data) 

Inaccessible Arroyo San Augustin 462 2002 

Inaccessible Arroyo El Bulito 640 2002 

Inaccessible Canada del Agua 570 2002 

X X Canada de Santa Anita 829 2001 

X Not accessed Sacate 2002 

Inaccessible Canada de la Cuarta 414 2002 

Inaccessible Canada de Alegria 925 2002 

 Agua Caliente 666 

X X Canada de la Gaviota 5216 2001 

X Extirpated Canada San Onofre 538 2002 

Dry Canada del Molino 399 2002 

X X Arroyo Hondo 1133 2001 

X Barrier Arroyo Quemado 779 2002 

X Barrier Tajiguas Creek 1613 2002 

X Extirpated Canada del Refugio 2107 2002 

X Barrier Canada del Venadito 520 2002 

X Barrier Canada del Corral 1701 2002 

X Extirpated Canada del Capitan 1607 2002 

Inaccessible Las Llagas Canyon 716 2002 

X Inaccessible Gato Canyon 911 2002 

Inaccessible Las Varas Canyon 748 2002 

X Barrier Dos Pueblos Canyon 2183 2002 

X Inaccessible Eagle Canyon 1204 2002 

X Barrier Tecolote Canyon 1467 2002 

X Barrier Bell Canyon 1589 2002 

X X Goleta Slough Complex 12393 2001 

X Barrier Arroyo Burro 2651 2002 

X X Mission Creek 2814 2000 

Barrier Sycamore Creek 1050 2002 

X X Montecito Creek 1719 2001 

X Barrier Oak Creek 389 2002 

X X San Ysidro Creek 1057 2002 

X X Romero Creek 1507 2001 

Inaccessible Toro Canyon Creek 953 2002 

X X Arroyo Paredon 1218 2000 

X Barrier Carpinteria Salt Marsh Complex 2636 2002 

X X Carpinteria Creek 3961 2000 

X Barrier Rincon Creek 3796 2002 

 Los Sauces Canyon 2980 

X X Ventura River 58666 2001 (C. Zimmerman, pers. comm.) 

 Hall Canyon 1884 

 Arundell Barranca 2829 

X X Santa Clara River 420965 2001 (M. McEachron, pers. comm.) 

 Calleguas Creek 89197 

Dry La Jolla Canyon 975 2002 

X Extirpated Big Sycamore Canyon 5523 2002 

 Little Sycamore Canyon 1538 

X X Arroyo Sequit 2839 2002 

X X Malibu Creek 28622 2002 

 Carbon Canyon 945 

Absent Las Flores Canyon 1075 2002 

Barrier Piedra Gorda Canyon 190 2002 
20 anadromous O. mykiss 

Historical 

Occurrence 

Current 

Occurrence 

Basin 

(north to south) Area (ha) Year (and external source for data) 

Barrier Pena Canyon 248 2002 

Absent Tuna Canyon 413 2002 

X X Topanga Canyon 5106 2002 (Dagit & Webb 2002) 

 Santa Ynez Canyon 1829 

 Santa Monica Canyon 4132 

Barrier Solstice Canyon 1154 2002 

Absent Corral Canyon 944 2002 

Dry Trancas Canyon 2372 2002 

Dry Escondido Canyon 933 2002 

Absent Ramirez Canyon 895 2002 

Absent Zuma Canyon 2405 2002 

 Ballona Creek 33981 

 Dominquez Channel 19579 

X Barrier Los Angeles River 216969 2002 

X Barrier San Gabriel River 183514 2002 

X Barrier Santa Ana River 661050 2002 

 San Diego Creek 31946 

Barrier Los Trancos Canyon 495 2002 

Barrier Muddy Canyon 376 2002 

Dry Moro Canyon 868 2002 

Inaccessible Emerald Canyon 581 2002 

 Laguna Canyon 2088 

Inaccessible Aliso Creek 9300 2002 

 Salt Creek 2543 

X Extirpated San Juan Creek 46125 2002 

 Canada de Segunda Deshecha 2966 

X X San Mateo Creek 34789 2002 (T. Hovey, pers. comm.) 

X Dry San Onofre Creek 11159 2002 (C. White, pers. comm.). 

 Las Pulgas Canyon 7087 

 Aliso Canyon 2350 

X Extirpated Santa Margarita River 190851 2000 (Swift et al. 2000) 

X Barrier San Luis Rey River2 146185 2002 

Inaccessible Loma Alta Creek 2558 2002 

Inaccessible Buena Vista Creek 5883 2002 

Inaccessible Agua Hedionda Creek 8037 2002 

 Canyon de las Encincas 1037 

 San Marcos Creek 15489 

Absent Escondido Creek 22289 2002 

Barrier San Dieguito River 90308 2002 

Absent Los Penasquitos Creek 24616 2002 

 Rose Canyon 10257 

 Tecolote Creek 2549 

X Barrier San Diego River 113741 2002 

 Chollas Creek 8755 

X Barrier Sweetwater River 59501 2002 

 Telegraph Canyon 2179 

X Barrier Otay River 37840 2002 

X Inaccessible Tijuana River 4149 2002 

Note: During the final phases of preparing this technical memorandum, we received reports of historic occurrence in Sacate Creek 

and Sycamore Creek.

 

2 Road crossing at mouth assumed to be migration barrier. 
 Contraction of the southern range limit 21 

 

Appendix C. Resident, residualized, or stocked O. mykiss above barriers. 

Data are for occurrence within the past 10 years, for historical steelhead basins with barrier exclusions or 

other extirpations. 

Basin Recent occurrence of O. mykiss

(north to south) Below lowest barrer

(this study) 

Above barrier 

(obs. 1994 – 2003) 

Source of observation 

Llagas Creek (Pajaro) Dry stream X J. Smith, pers. comm. 

San Antonio River (Salinas) Barrier exclusion X H. Fish, pers. obs. 

Nacimiento River (Salinas) Barrier exclusion X D. Boughton, pers. obs. 

Graves Creek (Salinas) Spot check (absent) 

Cayucos Creek Spot check (absent) 

Old Creek Barrier exclusion X Good et al. (2005) 

Jalama Creek Spot check (absent) X Stoecker & CCP (2002) 

Canada San Onofre Spot check (absent) Last obs. 1991 Stoecker & CCP (2002) 

Arroyo Quemado Barrier exclusion No recent records Stoecker & CCP (2002) 

Tajiguas Creek Barrier exclusion X Stoecker & CCP (2002) 

Canada del Refugio Spot check (absent) Last obs. 1990 Stoecker & CCP (2002) 

Canada del Venadito Barrier exclusion No recent records Stoecker & CCP (2002) 

Canada del Corral Barrier exclusion Last obs. 1993 Stoecker & CCP (2002) 

Canada del Capitan Spot check (absent) No recent records Stoecker & CCP (2002) 

Dos Pueblos Canyon Barrier exclusion X Stoecker & CCP (2002) 

Tecolote Canyon Barrier exclusion X Stoecker & CCP (2002) 

Bell Canyon Barrier exclusion No recent records Stoecker & CCP (2002) 

Arroyo Burro Barrier exclusion No recent records Stoecker & CCP (2002) 

Oak Creek Barrier exclusion No recent records Stoecker & CCP (2002) 

Carpinteria Salt Marsh Complex Barrier exclusion No recent records Stoecker & CCP (2002) 

Rincon Creek Barrier exclusion X Stoecker & CCP (2002) 

Big Sycamore Canyon Spot check (absent) no barriers? 

Los Angeles River Barrier exclusion X Good et al. (2005) 

San Gabriel River Barrier exclusion X Good et al. (2005) 

Santa Ana River Barrier exclusion X Good et al. (2005) 

San Juan Creek Spot check (absent) 

San Onofre Creek Dry stream no barriers? 

Santa Margarita River Absent X Good et al. (2005) 

San Luis Rey River Barrier exclusion X Good et al. (2005) 

San Diego River Barrier exclusion X Good et al. (2005) 

Sweetwater River Barrier exclusion X Good et al. (2005) 

Otay River Barrier exclusion Good et al. (2005) 

RECENT TECHNICAL MEMORANDUMS

Copies of this and other NOAA Technical Memorandums are available from the National Technical 

Information Service, 5285 Port Royal Road, Springfield, VA 22167. Paper copies vary in price. Microfiche 

copies cost $9.00. Recent issues of NOAA Technical Memorandums from the NMFS Southwest Fisheries 

Science Center are listed below:

NOAA-TM-NMFS-SWFSC-370 Ichthyoplankton and station data for surface (Manta) and oblique (Bongo)

 plankton tows for California Cooperative Oceanic Fisheries Investigations

 survey cruises in 2003.

 E.S. ACUÑA, R.L. CHARTER, and W. WATSON

 (March 2005)

371 Preliminary report to congress under the international dolphin conservation

 program act of 1997.

 S.B. REILLY, M.A. DONAHUE, T. GERRODETTE, P. WADE, L. BALLANCE,

 P. FIEDLER, A. DIZON, W. PERRYMAN, F.A. ARCHER, and E.F. EDWARDS

 (March 2005)

372 Report of the scientific research program under the international dolphin 

 conservation program act.

 S.B. REILLY, M.A. DONAHUE, T. GERRODETTE, K. FORNEY, P. WADE, 

 L. BALLANCE, J. FORCADA, P. FIEDLER, A. DIZON, W. PERRYMAN, 

 F.A. ARCHER, and E.F. EDWARDS

 (March 2005)

373 Summary of monitoring activities for ESA-listed Salmonids in California's

 central valley.

 K.A. PIPAL

 (April 2005)

374 A complete listing of expeditions and data collected for the EASTROPAC 

 cruises in the eastern tropical Pacific, 1967-1968.

 L.I. VILCHIS and L.T. BALLANCE

 (May 2005)

375 U.S. Pacific marine mammal stock assessment: 2004.

 J.V. CARRETTA, K.A. FORNEY, M.M. MUTO, J. BARLOW, J. BAKER

 and M.S. LOWRY

 (May 2005)

376 Creating a comprehensive dam dataset for assessing anadromous fish 

 passage in California. 

 M. GOSLIN

 (May 2005) 

377 A GIS-based synthesis of information on spawning distributions of 

 chinook ESU.

 A. AGRAWAL, R. SCHICK, E. BJORKSTEDT, B. SPENCE, M. GOSLIN

 and B. SWART

 (May 2005)

378 Using lidar to detect tuna schools unassociated with dolphins in the

 eastern tropical Pacific, a review and current status

 J.P LARESE

 (May 2005)

 

379 Predicting the potential for historical coho, chinook and steelhead

 habitat in northern California.

 A. AGRAWAL, R.S. SCHICK, E.P. BJORKSTEDT, R.G. SZERLONG, 

 M.N. GOSLIN, B.C. SPENCE, T.H. WILLIAMS, and K.M. BURNETT

 (June 2005)